Nitrogen rates, crop rotation and root health are proving to be the biggest factors affecting yield for southern region farmers according to interim results from the GRDC’s National Paddock Survey.

CSIRO group leader and farming systems scientist Dr Roger Lawes revealed these key yield gap variables at GRDC Research Updates earlier this year. He outlined preliminary results which showed for a given amount of rainfall the most dominant factors affecting yield were nitrogen nutrition and crop health, with weeds proving a lesser factor.

Consultants and researchers from farming systems groups across the country are now preparing for their fourth and final year of data collection for this GRDC investment led by Birchip Cropping Group (BCG) and CSIRO.

Their work will give farmers a region-by-region understanding of what variables they should be most concerned with in bridging the gap between their actual yield and water limited yield. This is defined as the maximum possible yield based on optimal sowing date, current cultivars and nutrients, pests, disease and weeds not limiting yield.

The National Paddock Survey will give farmers an insight into their key yield gap constraints relevant to each paddock as a result of four years of research across the country.

National Paddock Survey project lead and BCG consultant Harm van Rees says final results from the investment will equip farmers with a clear idea of the manageable factors limiting yield in their region based on results from their location.

“This research is trying to take previous yield gap analysis a step further and take it to individual paddocks rather than a soil type,” he says.

“We monitor two zones in each paddock and work out the size of the yield gap for each zone and then analyse the data for factors limiting production over a four year rotation.

“This analysis is only for factors that farmers can manage. For example, if a grower has subsoil limitations on their property with high salt levels then that can’t be changed as it is a physical limitation, but if they’ve got a disease level, then that can be managed.”

An example of a paddock yield map of a 2015 wheat crop showing the monitoring points of two zones used in the National Paddock Survey.

In the western, northern and southern GRDC regions across the Australian grain belt, 250 paddocks are being monitored, with 90 locations being monitored in the south. These farms were selected by local consultants and farm groups and represent the range of prevailing rainfall and soil conditions across the grain belt.

Consultants and farming systems groups monitor two zones within each NPS paddock which includes: deep soil cores prior to sowing which is analysed for soil N, soil water content and subsoil limitations. They also record tillage, stubble retention, crop type, cultivar, sowing date and monitor the crop for weeds, insects, diseases and take plant root samples which are analysed for root health. At the end of each season, crop yields are measured using a harvester’s yield monitor. The data collected for each season is reviewed at annual project meetings, to allow consultants and researchers to discuss insights and information regarding individual paddock performance.

Dr Lawes says results so far show the yield gap cannot be attributed to one factor but rather a number of factors as shown in table 1. The level of impact of each factor varies by region and in some cases the factors were driven by the first limiting constraint.

Table 1. The ranking (importance) of the variables that contribute to the size of the yield gap across the three regions

Region

In-crop rainfall

Nitrogen applied

Previous crop

Weeds

Root score

West

1

2

3

4

South

4

1

2

5

3

North

1

2

4

3

Dr Lawes says the survey has identified the yield gap, or the difference between actual and potential yield in wheat, was 1.1 tonnes per hectare in the northern region, 1.2t/ha in the southern region, and 1.3t/ha in the western region.

Frost and heat shock are yield limitations many farmers would identify as major concerns in the southern region. Mr van Rees says this is particularly the case as the timing of frost and heat shock events appears to be changing.

“Frost and heat shock are manageable in some ways through cultivar and sowing dates, but they can never be completely avoided,” he says.

“Most farmers are already managing many limiting factors and information from the National Paddock Survey will help them further with recognising their exposure to the most limiting factors and options for management.”